The invention concerns a process which makes possible without expensive uranium-thorium separation the return without loss of uranium-thorium wastes in the form of a stable uranium-thorium solution in the production-casting process. In this process no byproducts are formed which could cause an additional waste problem.
To prepare high temperature reactor-fuel elements there are needed (U,Th)O.sub.2 particles having defined uranium-thorium ratios and specific particle properties. These particles are formed by the kernel casting process under special conditions and are solidified by drying, sintering and coating, e.g., see German Auslegeschrift No. 1,542,346 and related Hackstein U.S. Pat. No. 3,535,264. The entire disclosure of the Hackstein U.S. patent is hereby incorporated by reference and relied upon.
The kernel casting solution consists of a uranyl nitrate-thorium nitrate solution and a polyvinyl alcohol solution which are mixed together in specific ratios shortly before the casting process. Viscosity, pH and ammonium nitrate content substantially influence the formation of kernels and particle shape and therefore must be held within defined limits.
Numerous processes are known for the preparation of the valuable (U,Th)O.sub.2 waste ocurring in the various production steps in the form of, e.g., powders, kernels or particles, practically all of which have as the object the separation of uranium and thorium and therewith the separate return of uranium and thorium.
Thereby there were used exclusively extractive processes which brought about a separation of uranium and thorium from different nitric acid solutions by means of tributyl phosphate (Swiss Pat. No. 442,257) or other extracting agents (AAEC-Report TM310). In other processes thorium was separated through oxalate precipitation and the uranium recovered by purification precipitation or extraction.
However, these processes have various disadvantages. First extraction processes are very expensive and require a large expenditure of time and apparatus. Furthermore, in the working up byproducts occur and therefore, a recovering of uranium and thorium without loss in the casting process is not possible.
A direct recovery of the uranium-thorium solution obtained by dissolving (U,Th)O.sub.2 scrap in nitric acid-hydrofluoric acid was not previously accomplished since in the neutralization of the 6-8N nitric acid solution with ammonia uranium and thorium are precipitated. However, this neutralization is necessary in order to be able to again employ the solution in the production process.